Nitride-Based Red-Emitting Phosphors in RGB (Red-Green-Blue) Lighting Systems

ABSTRACT

Embodiments of the present invention are directed to nitride-based, red-emitting phosphors in red, green, and blue (RGB) lighting systems, which in turn may be used in backlighting displays and warm white-light applications. In particular embodiments, the red-emitting phosphor is based on CaAlSiN 3  type compounds activated with divalent europium. In one embodiment, the nitride-based, red emitting compound contains a solid solution of calcium and strontium compounds (Ca,Sr)AlSiN 3 :Eu 2+ , wherein the impurity oxygen content is less than about 2 percent by weight. In another embodiment, the (Ca,Sr)AlSiN 3 :Eu 2+  compounds further contains a halogen in an amount ranging from about zero to about 2 atomic percent, where the halogen may be fluorine (F), chlorine (Cl), or any combination thereof. In one embodiment at least half of the halogen is distributed on 2-fold coordinated nitrogen (N2) sites relative to 3-fold coordinated nitrogen (N3) sites.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims priority toU.S. patent application Ser. No. 13/626,684 filed Sep. 25, 2012, whichis a continuation of, and claims priority to, U.S. patent applicationSer. No. 12/632,550 filed Dec. 7, 2009, now U.S. Pat. No. 8,274,215,which claims priority to U.S. Provisional Patent Application No.61/122,569 filed Dec. 15, 2008, and is a continuation-in-part of, andclaims priority to, U.S. patent application Ser. No. 12/250,400 filedOct. 13, 2008, which claims priority to U.S. Provisional PatentApplication No. 61/054,399 filed May 19, 2008. The disclosures of theaforementioned applications are all incorporated by reference herein intheir entirety.

FIELD OF THE INVENTION

Embodiments of the present invention are directed to nitride-based,red-emitting phosphors, which may be used in red, green, and blue (RUB)lighting systems, which in turn may be used in backlighting displays andwarm white-light applications. In particular embodiments, thered-emitting phosphor is based on (Ca, Sr, Ba)AlSiN₃ type compoundsactivated with divalent europium.

BACKGROUND OF THE INVENTION

There is a need in the field(s) of optical engineering for red, green,and blue lighting systems in various devices, applications, andtechnologies. Among these technologies are backlighting sources fordisplay systems, such as plasma displays, and warm-white light sourcesin general lighting.

Various configurations of LED light sources and light-emitting phosphorsare possible in the design of such RGB systems. The conventional one,shown schematically in FIG. 1A, employs three light emitting diodes(LEDs). Each of the three LEDs is a semiconductor-based integratedcircuit, or “chip”, and there is one chip for each of the red, green,and blue colors. A disadvantage of the conventional system is that aseparate electrical current controller is needed for each LED, like inmany situations, it is better to have as few current controllers in thesystem as is possible. The prior art system of FIG. 1A requires threecircuit controllers.

The RGB systems depicted in FIGS. 1B-D differ from that of FIG. 1A inthat they have at least one photoluminescent substance (a “phosphor”)substituting for at least one of the red, green, or blue LEDs of thesystem. The progression from FIGS. 1A-1D shows the effect of replacingLEDs with phosphors; in no particular order regarding color, first thegreen LED is replaced with a green phosphor (going from FIG. 1A to 1B);then the red LED is replaced with a red phosphor along with the green(going from FIG. 1B to 1C); finally, all three visible light emittingLEDs are replaced in FIG. 1D with phosphors, although a UV-emitting LEDhas been added to this system to provide excitation radiation to thephosphors. Thus, FIG. 1D depicts a strategy that is in a sense somewhatopposite to that of FIG. 1A, in that each of the RGB colors in FIG. 1Dis provided by a phosphor, and therefore the excitation source is anon-visible, UV emitting LED. The practice of technologies based on a UVexcitation source are somewhat further away from commercialization thanthose based on blue LEDs, but nonetheless, the configuration of FIG. 1Dis still one in which the present nitride-based red phosphors may beused.

This means that no blue phosphors are used in the embodiments of FIGS.1B and 1C because the blue LED provides the blue component of the lightneeded in various applications like backlighting and warm-white lightgeneral lighting. In this regard the blue LED is unique in the systembecause it serves a dual role; in addition to providing blue light tothe final light product, it provides excitation to either or both of thered or green phosphors in the system. Systems such as those depicted inFIGS. 1C and 1D are the subject of the present disclosure; theseconfigurations are particularly suited for silicon nitride basedred-emitting phosphors.

Since earlier versions of these red phosphors were based on nitrides ofsilicon, they may be generically referred to as “nitride-based”silicates, or nitridosilicates. Newer versions have included aluminumsuch that the resulting compounds are referred to as“nitridoaluminosilicate nitrides.” The deliberate inclusion of oxygeninto these crystals in a desired stoichiometric manner gives rise to acertain class of red-emitting phosphors, and compounds known as“SiAlONs” can also be in some cases sources of green and yellow-greenillumination. When oxygen substitutes for nitrogen the resultingcompound may be described as an “oxynitrides.”

As alluded to earlier, a combination of LED-generated blue light, andphosphor-generated green and red light, may be used to generate thewhite light from a so-called “white LED.” Previously known white lightgenerating systems used a blue LED in conjunction with a yellowemitting, cerium-doped, yttrium aluminum garnet known as “YAG,” havingthe formula Y₃Al₅O₁₂:Ce³⁺. Such systems have correlated temperatures(CCTs) of greater than about 4,500 K, and color rendering indexes (CRIs)ranging from about 75 to 82. The blue emitting LED provides excitationradiation ranging from about 400 to 480 nm.

One way of achieving a flexibility of design in blue LED-based devicesinvolves creating a wider separation between the yellow and/or greenphosphors, and the red phosphors, the phosphors relative to one anotherin CIE space. CIE coordinates will be discussed further later in thisdisclosure, but suffice it to say for now that “CIE space” means thearea in a triangle mapped by two vertices of a triangle defined byphosphors, and the third by the blue LED. A yellow and/or green apexwidely separated from that of the blue LED create a rich diversity ofcomponents for white light generation.

As described in U.S. Pat. No. 7,252,787 to D. Hancu et al., red sourceswere used with YAG and TAG-based yellow sources to produce a high colorrendering index have included nitrides having the formula(Ba,Sr,Ca)_(x)Si_(y)N_(z):Eu²⁺, where each of the x, y, and z parameterswas greater than zero. A disadvantage of such phosphors used withYAG/TAG was that they reabsorb emissions from those phosphors due tooverlapping of the Eu²⁺ absorption bands with the emission of the(Tb,Y)₃Al₅O₁₂:Ce³⁺) phosphors. Thus, there is a need for red phosphorshaving a redder emission than these nitrides to produce white lightillumination with high CRI.

Host lattices for new red-emitting phosphors based on nitridosilicatecompounds were introduced in the mid-1990's. Such phosphors havedesirable mechanical and thermal properties due to a three dimensionalnetwork of cross-linked SiN₄ tetrahedra in which alkali earth ions(M=Ca, Sr, and Ba) are incorporated. The formula used in U.S. Pat. No.6,649,946 to Bogner et al. to describe such phosphors wasM_(x)Si_(Y)N_(z), where M was at least one of an alkaline earth metal,and where z=2/x+4/3y. The nitrogen of these nitrides increased thecontent of colvalent bonding, and thus ligand-field splitting. This leadto a pronounced shift of excitation and emission bands to longerwavelengths in comparison to oxide lattices.

The effect of the alkaline earth component of such nitridosilicates wheny is 5 was investigated by Y. Q. Li et al. in “Luminescence propertiesof red-emitting M₂Si₅N₈:Eu²⁺ (M=Ca, Sr, Ba) LED conversion phosphors,”J. of Alloys and Compounds 417 (2006), pp. 273-279. Polycrystallinepowders were prepared by a solid state reaction mechanism. The crystalstructure of the Ca-containing member of this family was monoclinic withspace group Cc, whereas the Sr and Ba members were isostructral withorthorhombic space group Pmn2₁. There was a formation of a completesolid solution between the Sr and Ba end-members in the lattercompound(s).

As taught by Li et al., the excitation spectra are not substantiallydependent on the type of alkaline earth, but the position of theemission bands are. The peak emission bands for a 1 mole percentactivator concentration were 605, 610, and 574 nm, for M=Ca, Sr, and Ba.The shift in the emission band with the nature of the alkaline earth isdue to a difference in the Stokes shift for each of the members, wherethe Stokes shift gradually increases with the sequence Ca>Sr>Ba, andthis trend is predictable if one observes that the relaxation of the4f⁶5d¹ state becomes less restricted when the size of the alkaline-earthion decreases. Further, the Stokes shift increases for as the Euconcentration is increased in all cases.

US 2007/0040152 elucidated the difficulties in producing anitridosilicate based compound such as M₂Si₅N₈, MSi₇N₁₀, and MSiN₂,where M is at least one element selected from Mg, Ca, Sr, and Ba, etc.,and where the compound contains substantially no oxygen. This may beachieved, it is taught, by using as starting materials the nitrides ofthe alkaline-earth elements and the rare earth elements, but thesenitrides are difficult to obtain, expensive, and difficult to handle.These factors conspire to make nitridosilicate-based phosphors difficultto produce industrially. As stated by the reference: “the conventionalnitridosilicate-based compound has the following problems: (1) lowpurity due to the presence of a large amount of impurity oxygen, (2) lowmaterial performance of a phosphor caused by the low purity; (3) highcost; and the like.” The problems included low luminous flux andbrightness.

What is needed in the art are red-emitting phosphors in red, green, andblue (RGB) lighting systems for use in backlighting displays and warmwhite-light applications, where the red phosphors have high luminousflux and brightness. The present disclosure describes improvements inred-emitting phosphor based on CaAlSiN₃ type compounds activated withdivalent europium. In conjunction with phosphors emitting at otherwavelengths, it is believed the present embodiments provide generalillumination sources having higher CRIs and lower CCTs than thosecurrently available.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to nitride-based,red-emitting phosphors, which may be used in red, green, and blue (RGB)lighting systems. These RBG systems may in turn be used in backlightingdisplays and warm white-light applications. In particular embodiments,the red-emitting phosphor is based on (Ca,Sr,Ba)AlSiN₃ activated withdivalent europium, where the (Ca,Sr,Ba) nomenclature means any of thesealkaline earths may be used, in any proportion one relative to theothers. Strontium (Sr) may substitute for Ca in the formula, in anycombination of amounts; in one embodiment a complete solid solution ofcalcium and strontium containing compounds (Ca,Sr)AlSiN₃ compounds aredisclosed where the impurity oxygen content is less than about 2 percentby weight. The present nitride-based, red emitting compounds may furthercomprise a halogen, whose content ranges from greater than about zero toabout 2 atomic percent, which halogen in one embodiment may be selectedfrom the group consisting of F and Cl. It is believed the halogen mayprovide some sort of a gettering effect during synthesis, and this maybe the mechanism by which the oxygen impurity content is kept to lowlevels. In one embodiment of the present invention, at least half of thehalogen is distributed on 2-fold coordinated nitrogen (N2) sitesrelative to 3-fold coordinated nitrogen (N3) sites.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D show various configurations and ways of arranging LEDs andphosphors to provide RUB light for use in white LED and backlightingsituations;

FIGS. 2A and 2B are graphs of the emission wavelength andphotoluminescence (PL), respectively, of the changes that occur when thefluorine content x supplied by NH₄F is increased above various halogenbaseline levels; the three compounds beingCa_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01):F_(0.04+x) when the europium source isEuF₃; Ca_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01):Cl_(0.04)F_(x) when the europiumsource is EuCl₃; and Ca_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01):F_(x) when theeuropium source is Eu₂O₃;

FIGS. 3A and 3B are graphs showing the effects on the CIE coordinates xand y with the same additions of NH₄F to the three compounds of FIGS. 2Aand 2B;

FIGS. 4A and 4B are graphs of the emission wavelength andphotoluminescence (PL), respectively, of the changes that occur when thechlorine content x supplied by NH₄Cl is increased above various halogenbaseline levels; the three compounds beingCa_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01):F_(0.04)Cl_(x) when the europium sourceis EuF₃; Ca_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01):Cl_(0.04+x) when the europiumsource is EuCl₃; and Ca_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01):Cl_(x) when theeuropium source is Eu₂O₃;

FIGS. 5A and 5B are graphs showing the effects on the CIE coordinates xand y with the same additions of NH₄F to the three compounds of FIGS. 4Aand 4B;

FIGS. 6A and 6B are graphs of the emission wavelength andphotoluminescence (PL), respectively, of the changes that occur when thefluorine content x supplied by NH₄F is increased above various halogenbaseline levels; the three compounds beingCa_(0.16)Sr_(0.82)AlSiN₃Eu_(0.02):F_(0.04+x) when the europium source isEuF₃; Ca_(0.16)Sr_(0.82)AlSiN₃Eu_(0.02):Cl_(0.04)F_(x) when the europiumsource is EuCl₃; and Ca_(0.16)Sr_(0.82)AlSiN₃Eu_(0.02):F_(x) when theeuropium source is Eu₂O₃;

FIGS. 7A and 7B are graphs showing the effects on the CIE coordinates xand y with the same additions of NH₄F to the three compounds of FIGS. 6Aand 6B;

FIGS. 8A and 8B are graphs of the emission wavelength andphotoluminescence (PL), respectively, of the changes that occur when thefluorine content x supplied by NH₄Cl is increased above various halogenbaseline levels; the three compounds beingCa_(0.16)Sr_(0.82)AlSiN₃Eu_(0.02):F_(0.04)Cl_(x) when the europiumsource is EuF₃; Ca_(0.16)Sr_(0.82)AlSiN₃Eu_(0.02):Cl_(0.04+x) when theeuropium source is EuCl₃; and Ca_(0.16)Sr_(0.82)AlSiN₃Eu_(0.02):Cl, whenthe europium source is Eu₂O₃;

FIGS. 9A and 9B are graphs showing the effects on the CIE coordinates xand y with the same additions of NH₄F to the three compounds of FIGS. 8Aand 8B;

FIGS. 10A and 10B are graphs of photoluminescence as a function ofemission wavelength for both the R630 and R640 type of compounds,respectively, when no additional halogen is provided by a flux such asNH₄F or NH₄Cl;

FIG. 10C is a collection of emission spectra comparing the R630 and R640nitride-based red phosphors (the numbers in the designations indicatingroughly the peak emission wavelength of that particular phosphor),having formulas Ca_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01) (R630),Ca_(0.158)Sr_(0.815)AlSiN₃Eu_(0.023), (R640), to the phosphorCa_(0.157)Sr_(0.808)AlSiN₃Eu_(0.035) (R645), where EuF₃ was used as thesource of europium in each of the three compositions;

FIGS. 11A-B are graphs of emission wavelength and photoluminescence as afunction of Sr content for the phosphors having the formulaCa_(0.98−x)Sr_(x)AlSiN₃Eu_(0.02), where Eu₂O₃ is the europium source;

FIGS. 12A-B are the normalized emission spectra ofCa_(0.98−x)Sr_(x)AlSiN₃Eu_(0.02) and Ca_(0.98−x)Sr_(x)AlSiN₃Eu_(0.02)with x=0.82 and x=0, respectively, plotted in this way to show theeffect of a wavelength shift to shorter wavelengths with the Sr doping,and a greater brightness with Sr doping;

FIG. 13 shows the effect of doping the present nitride-based redphosphors with further amounts of an alkaline earth metal; specifically,Ca, Sr, and Ba replacing a portion of the Ca/Sr mix discussed inprevious paragraphs: here the compound under study had the formulaCa_(0.2)Sr_(0.74)AlSiM_(0.05)N₃Eu_(0.01), where M was Ca, Sr, Ba,respectively;

FIGS. 14A-B provide a collection of emission wavelengths, the raw datashown in FIG. 14A and a normalized version shown in FIG. 14B, for anitride-based red phosphor with a 5% level of a group IIIA (of theperiodic table) metal M, where M is in this example either boron orgallium;

FIGS. 15A-B are graphs showing peak emission wavelength andphotoluminescent changes as a function of Eu source and content, whereEu sources include the oxide, fluoride, and chloride salts of theeuropium metal; the compound chosen for this study beingCa_(0.2)Sr_(0.8−x)AlSiN₃Eu_(x), where x ranged from 0.005 to 0.015;

FIG. 16 is a collection of excitation spectra for the R630 compositionhaving the formula Ca_(0.2)Sr_(0.8)AlSiN₃Eu_(0.01), and the R640composition having the formula Ca_(0.158)Sr_(0.815)AlSiN₃Eu_(0.023);

FIG. 17 is an x-ray diffraction (XRD) pattern of one of the typicalcompositions of the present embodiments; the particular compoundexamined was Ca_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01):F;

FIGS. 18A-18E are exemplary spectra of light intensity versus wavelengthemitted by warm white luminescent systems.

DETAILED DESCRIPTION OF THE INVENTION

An introduction to the field of nitride-based, red emitting phosphorshas been provided by K. Uheda et al. in “Luminescence properties of ared phosphor, CaAlSiN₃:Eu²⁺, for white light-emitting diodes,” publishedin Electrochemical and Solid-State Letters, 9 (4) H22-H25 (2006). Thisreference explains how white light-emitting diodes (LEDs) are attractingincreasing attention due to their high efficiencies and long lifetimes.The earliest “white light LEDs,” consisting of a blue LED as anexcitation source and provider of blue light, and a yellow-emittingYAG:Ce³⁺ phosphor, suffered from low color rendering because of the lackof a red component. An earlier attempt at providing the desired redcomponent was made with an Eu²⁺ activated alkaline earth sulfide, whichhad the advantage of being excitable by blue light, but which alsosuffered from the potential to be degraded by atmospheric moisture dueto its hygroscopic nature. The next generation of red phosphorsexhibited an improvement in properties: these materials were alkalineearth silicon nitrides such as CaSiN₂:Eu²⁺ (presentnomenclature=“1-1-2,” after the stoichiometric amounts of the Si and N,respectively) and Ca₂Si₅N₈:Eu²⁺ (“2-5-8”), where, at least in the lattercase, other alkaline earth metal elements may be substituted for Ca. Thestate of the art in red nitrides is the compound CaAlSiN₃:Eu²⁺(“1-1-1-3”), where again Ca can be any alkaline earth or mixture ofalkaline earths. These compounds have greater chemical stability andoptical efficiency than their 1-1-2 and 2-5-8 counterparts. The hostmaterial CaAlSiN₃ was already known from the CaO, AlN, and Si₃N₄ phasediagram.

The position of the 5d excitation bands of the Eu²⁺ ions in theseCaAlSiN₃ compounds at low energies is attributable to the influence ofthe highly covalent nature of the europium on an alkaline earth metalsite with nitrogen atoms, leading to a large crystal field splitting, aswell, also due to the presence of nitrogen. The covalency of the Eu—Nbond and the resultant crystal field strength around the Eu²⁺ ions issimilar in each of the members of this series (e.g., Ca, Sr, andBa-based), despite the fact there are two M sites with differentsymmetries. Of particular importance to white LEDs is the fact thesecompounds have efficient excitation in the same spectral region (400 to470 nm), matching the radiative blue light from an InGaN-based LED,which emits around 465 nm. Their broad-band emissions are due to a4f⁶5d¹→4f⁷ transition within the Eu²⁺ ion, with the Ca compound emittingat wavelengths ranging from about 605-615 nm, Sr at 609-680 nm, and Baat 570-680 nm. For M₂Si₅N₈:Eu²⁺ with M restricted to Sr and Ba, theemission band of Eu²⁺ successively shifts from orange when M is Sr, andyellow when M is Ba at low Eu concentrations, to red (up to 680 nm) forhigh concentrations of Eu.

While the advent of new red nitrides was crucial to the evolution of thestate of the art of backlighting and warm white light (specifically,their ability to impart a high color rendering to the product whitelight), their inclusion in a mix of light from other phosphors has bedeveloping over a period of time. The assignee of the presentapplication has been involved in this field for some time, and hascontributed a number of innovations to the art. The following tablelists several patents and published applications directed RUB systems,each patent (or published application) being incorporated herein in itsentirety. Table 1 is provided for the purpose of summarizing the red andgreen phosphor compositions that have contributed to the present stateof the art, and also to show the progression towards adoption of anCaAlSiN₃:Eu²⁺-type compounds in the present art.

U.S. Pat. No. 7,311,858, titled “Silicate-based yellow-green phosphors,”filed Sep. 22, 2004, discloses the combination of a blue LED, a greenphosphor that includes (but is not limited to) compositions having theformula M₂SiO₄:Eu²⁺D, where M is a divalent metal that includes thealkaline earths, and D is a dopant selected from the group consisting ofF, Cl, Br, I, P, S, and N, and a red phosphor that includes (but is notlimited to) the nitridosilicate (Sr,Ba,Ca)₂Si₅N₈:Eu²⁺.

U.S. Pat. No. 7,575,679, titled “Silicate-based green phosphors,” Nov.8, 2005, discloses the combination of a blue LED and the nitridosilicate(SrBaCa)₂Si₅N₈:Eu²⁺ (among other compounds) as the red phosphor, butthis patent discloses a slightly different green phosphor from that ofthe patent in the paragraph above. The green phosphors of U.S. Pat. No.7,575,679 may be described by the formula(Sr,M²⁺)_(x)(Si,P)(O,F,Cl)_(2+x):Eu²⁺, where M²⁺ again includes thealkaline earth metals. These green phosphors may be undoped; in otherwords, the amounts of P, F, and/or Cl in the phosphor may be zero.

U.S. Application No. 2008/0116786, title “Novel silicate-basedyellow-green phosphors,” filed Dec. 24, 2007 and a Continuation-in-partof U.S. Pat. No. 7,311,858, discloses the combination of a blue LED andgreen phosphor having the formula M₂SiO₄:Eu²⁺D (where M is a divalentmetal that includes the alkaline earths, and D is a dopant selected fromthe group consisting of F, Cl, Br, I, P, S, and N), as in U.S. Pat. No.7,311,858, but the red phosphor of the RGB solution includes thealuminonitridosilicate (Sr,Ba,Ca)AlSiN₃:Eu²⁺. The present embodimentsare directed to a more thorough disclosure of the use of thesealuminonitridosilicate-based red phosphors (which may also be callednitride-based red phosphors).

A further disclosure filed by the assignee of the present applicationcontributing to the RUB art is U.S. application Ser. No. 12/250,400,titled “Nitride-based red phosphors,” and filed Oct. 13, 2008, but basedon a provisional U.S. Application No. 61/054,399, title“Nitridosilicate-based red phosphors,” filed May 19, 2008. Theyellow-green phosphors of these two disclosures were based on thegeneral formula M₂SiO₄:Eu²⁺, which may be undoped, and used incombination with the blue LED and red phosphors of thenitridosilicate-based family of the type (Sr,Ba,Ca)AlSiN₃:Eu²⁺ discussedabove.

TABLE 1 Case ID Filing date Green phosphor Red phosphor U.S. Pat. No.7,311,858 Sep. 22, 2004 M₂SiO₄:Eu²⁺D Includes M includes alkaline earths(Sr,Ba,Ca)₂Si₅N₈:Eu²⁺ D = F, Cl, Br, I, P, S, and N U.S. Pat. No.7,575,697 Nov. 8, 2005 (Sr,M²⁺)_(x)(Si,P)(O,F,Cl)_(2+x):Eu²⁺ IncludesM²⁺ includes alkaline earths (Sr,Ba,Ca)₂Si₅N₈:Eu²⁺ (can be undoped; theamounts of P, F, Cl zero) U.S. Pub. 2008/0116786 Dec. 24, 2007M₂SiO₄:Eu²⁺D Includes M includes alkaline earths (Sr,Ba,Ca)AlSiN₃:Eu²⁺ D= F, Cl, Br, I, P, S, N and B U.S. Appl. No. 61/054,399 May 19, 2008M₂SiO₄:Eu²⁺ Includes U.S. Appl. No. 12/250,400 Oct. 13, 2008 M includesalkaline earths (Sr,Ba,Ca)AlSiN₃:Eu²⁺

The present disclosure will be divided into the following sections:first, a chemical description of the present red nitrides(stoichiometric formulas) will be given, followed by a description ofthe synthesis, focusing on the starting materials. The structure of thepresent nitride-based red phosphors will then be discussed in detail,although further reference to structure will be made later in thedisclosure with experimental x-ray diffraction (XRD) data. Experimentaldata comprising wavelength and photoluminescent changes upon inclusionof halogen will be presented, and reference will be made to the loweringof the oxygen content as a result of halogen inclusion. Finally, therole the present red nitrides may play in white light illumination andbacklighting applications will be presented with exemplary data.

Chemical Description of the Present Red Nitrides

There are several ways to describe the formula of the present phosphors.In one embodiment, the present phosphors have the form M-A-B-(N,D):Z,where M, A, and B are three cationic metals and/or semimetals withdivalent, trivalent, and tetravalent valences, respectively; N isnitrogen (a trivalent element), and D is a monovalent halogen that alongwith the nitrogen contributes to the anionic charge balance. Thus, thesecompounds may be thought of as halogen-containing nitrides. The elementZ is an activator in the host crystal, providing the photoluminescentcenters. Z may be a rare earth or transition metal element.

The present nitride-based red phosphors may be described in a slightlydifferent format, to emphasize the approximate ratios of the constituentelements. This formula takes the form M_(m)M_(a)M_(b)(N,D)_(n):Z_(z),where the stoichiometry of the constituent elements (m+z):a:b:n followsthe general ratios 1:1:1:3, although deviations from these integervalues are contemplated, and n may range from about 2.5 to about 3.5,endpoints inclusive. It is noted the formula shows that the activator Zsubstitutes for the divalent metal M_(m) in the host crystal, and thatthe host material of the phosphor contains substantially no oxygen (orat least, less than about 2 percent by weight, according to the presentembodiments).

Embodiments of the present invention are directed to nitride-based, redphosphors having the formula M_(a)M_(b)M_(c)(N,D)_(n):E_(z), where M_(a)may be a single divalent element, or it may be a combination of two ormore divalent elements (or two divalent elements used simultaneously).Divalent elements include elements from the second column of theperiodic table, the alkaline earth metals. The two divalent metals maybe, for example, Ca and Sr. In this formula, n may range from about 2.5to about 3.5, endpoints inclusive, and charge compensation may beaccomplished by a redistribution of cationic content, changes in numbersof vacancies, inclusion of impurities, and the like. In the presentphosphors, M_(a) may be a combination of at least two divalent alkalineearth metals selected from the group consisting of Mg, Ca, Sr, Ba; M_(b)is a trivalent metal such as Al, Ga, Bi, Y, La, and Sm; and M_(e) is atetravalent element such as Si, Ge, P, and B; N is nitrogen, and D is ahalogen such as F, Cl, or Br. The following description contains first adisclosure of how the starting materials containing the alkaline earthmetals may be prepared, then a description of the process by which thepresent nitride-based phosphors may be prepared, and then concludes withtesting results. The preparation of some of the starting materialsappears to be novel, as the inventors do not believe that one of the,strontium nitride, is commercially available.

The present nitride-based red phosphors may be described in yet anothermanner, this format emphasizing the stoichiometric relationship betweenthe amounts of the metals and halogen(s) present relative to the amountof nitrogen present in the nitride host. This representation has theform M_(m)M_(a)M_(b)D_(3w)N_([(2/3)(m+z)+a+(4/3)b−w])Z_(z). Theparameters m, a, b, w, and z fall within the following ranges:0.01≦m≦1.5; 0.01≦a≦1.5; 0.01≦b≦1.5; 0.0001≦w≦0.6, and 0.0001≦m≦0.5.

The metal M_(m) may be an alkaline earth or otherwise divalent metalsuch as Be, Mg, Ca, Sr, Ba, Zn, Cd, and/or Hg. Different combinationsare possible, and M_(m) may be a single one of these elements, or amixture of any or all of them. In one embodiment, the metal M_(m) is Ca.

M_(a) is a trivalent metal (or semimetal) such as B, Al, Ga, In, Y, Sc,P, As, La, Sm, Sb, and Bi. Again, different combinations and contents ofthese metals/semimetals are possible, and in one embodiment, the metalM_(a) is Al.

M_(b) is a tetravalent element such as C, Si, Ge, Sn, Ni, Hf, Mo, W, Cr,Pb, Ti, and Zr. In one embodiment, the tetravalent element M_(b) is Si.

The element D is a halogen such as F, Cl, or Br in this nitride-basedcompound, and may be contained within the crystal in any of a number ofconfigurations: for example, it may be present in a substitutional role(substituting for nitrogen) in the crystalline host; it may be presentinterstitially in the crystal, and/or perhaps within grain boundariesthat separate crystalline grains, regions, and/or phases. The amount ofthe halogen may range from about zero to about 2 atomic percent. Inother embodiments, the amount of the halogen ranges from about zero toabout 0.2, 0.5, 1, and 5 atomic percent, respectively.

Z is an activator comprising at least one or more of the rare earthelements and/or transition metal elements, and include Eu, Ce, Mn, Tb,and Sm. In one embodiment the activator Z is europium. According to oneembodiment of the present invention the activator is divalent, andsubstitutes for the divalent metal M_(m) in the crystal. The relativeamounts of the activator and the divalent metal M_(m) may be describedby the molar relationship z/(m+z), which falls within the range of about0.0001 to about 0.5. Keeping the amount of the activator within thisrange may substantially avoid the so-called quenching effect manifestedby a decrease in emission intensity caused by an excessive concentrationof the activator. The desired amount of the activator may change withthe particular choice of activator.

Starting Materials for the Present Synthesis

Prior art starting materials have typically consisted of the nitridesand oxides of the metals. For example, to produce the phosphorCaAlSiN₃:Eu²⁺ in U.S. Pat. No. 7,252,788, it is taught that the nitridestarting materials for the calcium, aluminum, and silicon sources may beCa₃N₂, AlN, and Si₃N₄, respectively. The source of the europium in thisdisclosure was the oxide Eu₂O₃. In contrast, the sources of the metalsin the present phosphors may be at least in part the halides of themetals, and typical examples include MgF, CaF, SrF, BaF, AlF, GaF, BF,InF, and (NH₄)₂SiF₆. The europium may be supplied by either of the twofluorides EuF₂ and EuF₃. The use of halides of the divalent, trivalent,and tetravalent metals is not the only way to supply the halogen to thephosphor: an alternative method is to use a flux such as NH₄F or LiF.

Specifically, compounds of the divalent metal M_(m) appropriate as rawmaterials in the synthesis of the present phosphors include nitrides,oxides, and halides; e.g., Mm₃N₂, MmO, MmD₂, where again D is F, Cl, Br,and/or I. Analogous raw material compounds of the trivalent metal M_(a)are MaN, Ma₂O₃, and MaD₃. The tetravalent metal starting compoundsinclude Mb₃N₄, and (NH₄)₂MbF₆. Compounds of the halide anion D includeNH₄D and AeD, where Ae is an alkaline metal such as Li, Na, and MD₂,where Me is an alkaline earth metal such as Mg, Ca, etc.

Prior art references have disclosed the oxide of europium, Eu₂O₃, as thesource of the europium activator, as this material is a readilyavailable commercial compound. The present inventors have discovered,however, that the oxygen in this compound has a deleterious effect onthe photoluminescent properties of the phosphor. One way of eliminatingthis problem is to use a europium source that does not contain oxygen,such as the substantially pure Eu metal, but this is a very expensiveapproach that is difficult to implement. One embodiment of the presentinvention is to use a Eu halide, such as EuF₃ and/or EuCl₃ aseuropium-containing starting materials. The present inventors have foundthat when a europium halide such as EuF₃ is used as the europium source,the emission efficiency of the phosphor increases, and the emissionwavelength of the phosphor shifts to a longer wavelength. Thus oneembodiment of the present invention is to use a europium compound EuD₃(D=F, Cl, Br, I), and not Eu₂O₃, as the europium source. These conceptswill be illustrated, and discussed more fully, in conjunction with theaccompanying figures.

The strontium nitride starting material bay be synthesized by nitridingSr metal under a nitrogen atmosphere at temperature of about 600-850° C.for about 5-12 hours. The resulting Sr nitride is pulverized in a glovebox within an inert atmosphere, such as a nitrogen atmosphere. Thechemical reaction used to prepare the Sr nitride starting material maybe represented by the following equation:

3Sr+N₂→Sr₃N₂

Ca nitride may be either obtained commercially or specially prepared. Ifit is desired to prepare one's own Ca nitride, then a similar proceduremay be used as that described above to prepare strontium nitride:calcium metal is nitrided under a nitrogen atmosphere at temperature ofabout 600-950° C. for about 5-12 hours. Note that the upper temperatureof the heating step is slightly higher in the Ca case than it was forthe Sr case. The Ca nitride that is obtained from that step ispulverized in a glove box under an inert atmosphere such as a nitrogenatmosphere. The chemical reaction may be represented by the followingequation:

3Ca+N₂→Ca₃N₂

The synthesis process of the new phosphors containing two or more of thedivalent element (such as Ca and Sr) is similar to that which wasdescribe in co-pending application titled “Nitride-based red phosphors,”filed Oct. 13, 2008, having application Ser. No. 12/250,400. applicationSer. No. 12/250,400 is incorporated herein in its entirety. In thepresent case, the raw materials Sr₃N₂, Ca₃N₂, AlN, Si₃N₄, and theEu-containing materials such as EuF₃, EuCl₃, Eu₂O₃, and/or theircombinations, were sealed under an inert atmosphere such as nitrogen andthe like. A glove box containing an inert atmosphere may be used. Theseraw materials are weighed, and then mixed using any of the methods knownin the art, such as mixing with an ordinary ball mill.

The raw material mixture is then fired at an elevated temperature in aninert atmosphere; a convenient way to perform this firing step is toplace the raw material mixture in a crucible, and then to place thecrucible in a tube furnace. The raw material mixture within the crucibleis heated to a temperature of about 1400-1700° C. using a heating rateof about 10° C. per minute, again, the entire procedure carried out inan inert atmosphere such as nitrogen or the like. Once at temperature,the mixture is maintained at 1400-1700° C. for about 2-10 hours tosinter the raw material mixture. After the sintering is complete, thesintered material is cooled to about room temperature, and thenpulverized using any pulverizing means known in the art such aspulverizing with a mortar or ball mill. The pulverizing step produces aphosphor in powder form having the desired composition.

Structure of the Present Nitride-Based Red Phosphors

Aluminonitridosilicates may be derived from nitridosilicates bysubstitutions of aluminum for silicon. A red phosphor having the formulaCaAlSiN₃ has been described by K. Uheda et al. in the reference alludedto earlier: “Luminescence properties of a red phosphor, CaAlSiN₃:Eu²⁺,for white light-emitting diodes,” in Electrochemical and solid-stateletters, 9 (4) (2006), pages H22-H25. The crystal structure of theCaAlSiN₃:Eu²⁺ family of compounds was found to be orthorhombic with aCmc2₁ space group, where the unit cell volume expanded linearly with anincrease in Eu concentration up to at least 20 mole percent. Thestructure is made up of tetrahedra of [SiN₄] and [AlN₄] forming cornersharing six-member rings; rings are combined to form sheets, of whichthere are two types, overlaid in an alternate fashion to form the threedimensional network. The Ca²⁺ ions accommodated in cavities in theoverlaid planes, and the Eu²⁺ ions substitute for the Ca²⁺ ions. Theoverlay of the two sheets, rotated 180 degrees one to the other, formsthe basis for the existence of two types of nitrogen sites, whichsignificance to the present embodiments will be made apparent from thenext several paragraphs.

FIG. 2 of the Uheda et al. reference shows that in any one of the twosheets, either Sheet-A or Sheet-B as labeled in the figure, a nitrogensite at any particular vertex (corner) of either a [SiN₄] or [AlN₄]tetrahedron is shared by only one other tetrahedron (again, either[SiN₄] or [AlN₄] tetrahedra), when the two-dimensional sheets are viewedas separate structures. But when the two types of sheets are overlaid,and bonding between the sheets is considered, there is created a secondtype of nitrogen site in which that vertex is corner shared with twoother tetrahedrons. The nature of the corner sharing is such that twothirds of the nitrogen sites are coordinated with three other Si/Altetrahedron, and the remaining one third of the N sites are coordinatedwith two other Si/Al tetrahedron. Uheda et al. point out that this is tobe contrasted with the nitridosilicate phosphor CaSiN₂:Eu²⁺ describedearlier, where all the N atoms are coordinated with only two Sitetrahedra. As a result, CaAlSiN₃:Eu²⁺ has a more rigid structure thanCaSiN₂:Eu²⁺.

Determination of just where the halogen atoms are located in the presentred nitrides, whether the halogen is a Cl, F atom, or combination ofboth Cl and F in the same phosphor, is best understood by consideringthe atomic arrangements of all the atoms within the CaAlSiN₃ crystalstructure. This topic has been reviewed by R-J Xie et al. in“Silicon-based oxynitride and nitride phosphors for white LEDs—areview,” published in Science and Technology of Advanced Materials 8(2007), pp. 588-600. The atomic arrangements in the materials,particularly as they pertain to a halogen in a CaAlSiN₃ crystal, will becovered in the next several paragraphs of the present disclosure.CaAlSiN3 itself has an orthorhombic crystal structure having the spacegroup Cmc2₁, and unit cell parameters of a=9.8007 Å, b=5.6497 Å,c=5.0627 Å.

The structure of the present CaAlSiN₃-based halogen-containing materialsis built up corner sharing SiN₄ and AlN₄ tetrahedra, linked in twodifferent ways: one third of the nitrogen atoms in so-called N2 sitesare linked to two SiN₄ or AlN₄ tetrahedral neighbors, and the remainingtwo thirds of the nitrogen atoms in N3 sites are connected to three SiN₄or AlN₄ tetrahedral neighbors. The Si⁴⁺ and Al³⁺ cations are randomlydistributed within the tetrahedral sites formed by four nitrogen atoms.Tetrahedra display corner sharing to form vertex-linked M6N18 rings,where M represents the aluminum and silicon cations. The Ca atoms residein the tunnels surrounded by six corner-sharing Al/Si occupiedtetrahedral, and are coordinated to two to four nitrogen atoms, wherethe average Ca—N bond length is 2.451 Å.

Effects of Halogen Content on Optical Properties

That halides of europium may be used as the source of the europium hasbeen taught by Hirosaki et al. (US 2007/0007494). Their disclosurestates that: “ . . . from the viewpoint of good reactivity with othernitride materials, oxides, nitrides, and halides of [europium] arepreferred, and oxides are particular preferred since the raw materialsare available at a low cost and the temperature for phosphor synthesiscan be lowered. This patent application goes on to disclose that Euhalides such as EuF₂, EuF₃, EuCl₂, and EuCl₃ are [also] preferred sincethey have an effect of accelerating crystal growth. Eu₂O₃ isparticularly preferred, because it is an inexpensive raw material, has alow degree of deliquescency, and enables a synthesis of a high-luminancephosphor at a relatively low temperature. The amounts of the halogenfrom this source that end up inside the crystal, if any, are notdiscussed, and certainly the benefits of such a halogen content (such asthe potential to getter contaminant oxygen) are not given.

The effect of the halogen content in the family of CaAlSiN₃ phosphors onphotoluminescence and chromaticity has not, to the present inventors'knowledge, been disclosed in the literature. In the ensuing discussion,photoemission intensity, peak emission wavelength, and the chromaticityparameters x and y were measured as a function of halogen content, thiscontent providing a baseline level supplied by a halogenated europiumsource. A control was run for each experiment, for which the phosphorwas synthesized using Eu₂O₃ as the europium source, and thus the controlsamples had no baseline halogen content. In each of the graphs of FIGS.2-9 there are three curves, one for each of the three europium sourcesEu₂O₃, EuCl₃, and EuF₃. Thus, the curve from the Eu₂O₃ synthesizedphosphor has no baseline halogen; the curve from the EuCl₃ synthesizedphosphor has a starting level of chlorine of x equal to about 0.04.

Additional halogen “x” (see formulas) was supplied in the form of NH₄F(FIGS. 2, 3, 6, 7) or NH₄Cl (FIGS. 4, 5, 8, 9), the halogen from thissource being the independent variable in the experiment. It wasincreased from zero to about 0.3, stoichiometrically, to see what effectthis had on the optical properties mentioned above. By this method, aparticular red nitride of the present embodiments may contain both thehalogens F and Cl in its composition, one or the other, or neither ofeach.

The first set of data (FIGS. 2-5) is for an exemplary phosphor havingthe formula Ca_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01)(F,Cl)_(0.04+x) when theeuropium source is EuF₃ and EuCl₃, respectively, andCa_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01)(F,Cl), when the europium source (Eu₂O₃)provides no halogen. The phosphors in this family may collectively bedesignated R630 for their emission color (R=red) and wavelength (in nm).The second set of data (FIGS. 5-8) was generated by analogousexperiments on the exemplary phosphorsCa_(0.16)Sr_(0.82)AlSiN₃Eu_(0.02)(F,Cl)_(0.04+x) andCa_(0.16)Sr_(0.82)AlSiN₃Eu_(0.02)(F,Cl)_(x), again where the compositioncontains F if the europium source was EuF₃; Cl if the europium sourcewas EuCl₃; and no halogen when the europium source was the oxide ofeuropium (Eu₂O₃). The phosphors in this family may collectively bedesignated R640, again for their red emission color, this time centeredat a wavelength of about 640 nm. The oxygen content of the R630 group ofphosphors was about 1 weight percent, whereas that for the R640 familyabout 1.35 weight percent.

FIGS. 2A and 2B are graphs of the emission wavelength andphotoluminescence (PL), respectively, of the changes that occur when thefluorine content x supplied by NH₄F is increased above various halogenbaseline levels; the three R30 compounds beingCa_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01):F_(0.04+x) when the europium source isEuF₃; Ca_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01):Cl_(0.04)F_(x) when the europiumsource is EuCl₃; and Ca_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01):F_(x) when theeuropium source is Eu₂O₃. It may been seen that for this particular setof compounds, emission wavelength increased as more and more fluorinewas added from the NH₄F source, and thus the emission became more red incolor. This property is viewed as being especially beneficial when theuser is attempting to engineer phosphors having a desired chromaticitywith particular color rendering properties. The effect of an increase inwavelength with additional halogen is the same no matter what the natureof the baseline halogen: F, Cl, or no initial halogen in the case of thecontrol (Eu₂O₃). This beneficial effect comes at the expense of a slightloss of photoluminescent intensity (PL decreases as additional halogenabove the baseline is incorporated into the crystal), although it isnoted that this decrease is not substantial, and many situations arecontemplated where it is well worth tolerating in order to achieve thedesired chromaticity and color rendering.

FIGS. 3A and 3B are graphs showing the effects on the CIE coordinates xand y with the same additions of NH₄F to the same three compounds as inFIGS. 2A and 2B. Because of the importance the present embodiments hason chromaticity, it is viewed appropriate to provide a short discussionof what is meant by chromaticity, and the implications of that propertyon other issues affecting optical performance, such as color temperatureand color rendering.

As described by K. Narisada et al. in “Color Vision,” Chapter 17,Section One of the Phosphor Handbook (CRC Press, New York, 1999), pp.799-818, the CIE colorimetric system is derived from three imaginaryreference color stimuli (called tristimulus values), generated bycomparing a reference (monochromatic wavelength) color with a test colormade by the mixing of three primary colors. Test light source colors arespecified from the tristimulus values, and used to define thechromaticity coordinates x and y of the light sources. A plot of the xand y coordinates of every color designated by the Intersociety ColorCouncil-National Bureau of Standard (ISCC-NBS) is called the CIEchromaticity diagram, shown in the Phosphor Handbook at page 809.

K. Narisada et al. further teach the concept of chromatic adaptation,the function of vision that minimizes the influence of the (variable)color of the illumination on the perception of the object beingilluminated. Variable color illumination means that the chromaticitycoordinates of the light illuminating the object do not alwayscorrespond to the perceived colors. One method (proposed by the CIE) ofcorrecting for this lack of correspondence is to objectively define thecolor of light by “color temperature,” the absolute temperature of ablack body radiator radiating light with a color the same as the lightsource. The color of the light source changes from a reddish to a bluishcolor as the color temperature increases. The “correlated colortemperature” is the absolute temperature of a black body radiatorclosest to the light source when the chromaticity coordinates of thelight source do not precisely match the radiator.

A final property of color vision useful in describing the presentnitride-based red phosphors is color rendering, a property of the lightsource that changes the colors of the object illuminated by that source.The “color rendering index,” represented by the parameter R_(a),indicates the extent of the color rendering properties of a lightsource. It is calculated by taking the average of the differences indistance between chromaticity points of the sample and the referencelight source for eight different selected object colors. The maximumvalue of R_(a) is 100, which means there is no difference between sampleand reference source for any of the eight selected colors.

The color temperature and color rendering index values of exemplarynitride-based phosphors of the present embodiments will be discussedlater in a section addressing white light illumination, but the effectsof halogen addition the CIE coordinates x and y will be addressed atthis time. FIGS. 3A and 3B are graphs showing the effects on the CIEcoordinates x and y (respectively) with the same additions of halogenfrom NH₄F to the same three compounds as in FIGS. 2A and 2B. It will beobserved that the x coordinate increases, and they y coordinatedecreases, confirming the shift to longer wavelengths and a deeper redcolor when viewing the CIE chromaticity diagram of the PhosphorHandbook.

FIGS. 4A and 4B are graphs of the emission wavelength andphotoluminescence (PL), respectively, of the changes that occur when thechlorine content x supplied by NH₄Cl is increased above various halogenbaseline levels; the three compounds beingCa_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01):F_(0.04)Cl_(x) when the europium sourceis EuF₃; Ca_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01):Cl_(0.04+x) when the europiumsource is EuCl₃; and Ca_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01):Cl_(x) when theeuropium source is Eu₂O₃. The trends are the same as those for FIGS. 2Aand 2B; that is to say, emission wavelength increased as more and morefluorine was added from the NH₄F source, and thus the emission becamemore red in color.

FIGS. 5A and 5B are graphs showing the effects on the CIE coordinates xand y (respectively) with the same additions of halogen from NH₄Cl tothe same three compounds as in FIGS. 4A and 4B. It will be observedthat, as in the case with the R630 compounds for which NH₄F was added,the x coordinate increases, and they y coordinate decreases, confirmingthe shift to longer wavelengths and a deeper red color when viewing theCIE chromaticity diagram.

An analogous set of experiments was carried out with a family of R640compounds; these results are shown in FIGS. 6-9. FIGS. 6A and 6B aregraphs of the emission wavelength and photoluminescence (PL),respectively, of the changes that occur when the fluorine content xsupplied by NH₄F is increased above various halogen baseline levels; thethree compounds being Ca_(0.16)Sr_(0.82)AlSiN₃Eu_(0.02):F_(0.04+x) whenthe europium source is EuF₃;Ca_(0.16)Sr_(0.82)AlSiN₃Eu_(0.02):Cl_(0.04)F_(x) when the europiumsource is EuCl₃; and Ca_(0.16)Sr_(0.82)AlSiN₃Eu_(0.02):F_(x) when theeuropium source is Eu₂O₃. It may been seen that for this particular setof compounds, emission wavelength again increases as more fluorine isadded from the NH₄F source, and thus the emission becomes more red incolor. The photoluminescence graph shows that photoluminescence is notaffected as adversely as it was in the case of the R630 compound. CIEchanges for those compounds are shown in FIGS. 7A and 7B.

FIGS. 8A and 8B are of the emission wavelength and photoluminescence(PL), respectively, of the changes that occur when the chlorine contentx supplied by NH₄Cl is increased above various halogen baseline levels;the three compounds beingCa_(0.16)Sr_(0.82)AlSiN₃Eu_(0.02):F_(0.04)Cl_(x) when the europiumsource is EuF₃; Ca_(0.16)Sr_(0.82)AlSiN₃Eu_(0.02):Cl_(0.04+x) when theeuropium source is EuCl₃; and Ca_(0.16)Sr_(0.82)AlSiN₃Eu_(0.02):Cl_(x)when the europium source is Eu₂O₃. Following the trend set by the R630compounds, these R640 compounds show an increase in emission wavelengthas chlorine is added, and photoluminescent intensity decreases somewhatas well. CIE changes for those compounds are shown in FIGS. 9A and 9B.

Photoluminescence as a function of emission wavelength for both the R630and R640 type of compounds, when no additional halogen is provided by aflux such as NH₄F or NH₄Cl, is shown in FIGS. 10A-B. In both cases (R630and R640) the fluorinated versions demonstrated the highest intensities,followed by the chlorinated versions. The non-halogenated versions(provided by synthesizing R630 and R640 with the oxide of europium,Eu₂O₃, so that no halogen was added) showed the lowest intensities. Theorder of the compounds exhibiting the longest emission wavelength wasdifferent, however, with the non-halogenated compound having the longestwavelength in R630, and the shortest of the respective series in R640.

Since fluorine doping of R630 and R640 was shown to provide the highestphotoluminescent intensity (FIGS. 10-A-B), the two were compared on thesame graph, in FIG. 10C (notwithstanding a slight variation in the R640formula), along with another nitride-based phosphor having the formulaCa_(0.157)Sr_(0.808)AlSiN₃Eu_(0.035), designated R645. This nomenclaturecomes from the phosphor emitting at 645 nm; the reader will note in FIG.10C that it also had the lowest intensity when compared to R630 andR640. FIG. 10C is a collection of emission spectra of the nitride-basedred phosphors having the designations R630, R640, and R645, the numbersin the designations indicating roughly the peak emission wavelength ofthat particular phosphor. The formulas of those three phosphors are,respectively, Ca_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01) (R630),Ca_(0.158)Sr_(0.815)AlSiN₃Eu_(0.023), (R640), andCa_(0.157)Sr_(0.808)AlSiN₃Eu_(0.035) (R645), where EuF₃ was used as thesource of europium in each of the three compositions. This data setshows that photoluminesce intensity decreases with increasing peakemission wavelength.

In the experiments described above that investigated the influence of ahalogen inclusion, one will have observed that, to various degrees,strontium (Sr) was used to substitute for calcium (Ca). The effect ofstrontium substitution in a CaAlSiN₃ host will be investigated morethoroughly in the next section, along with a few other elementalsubstitutions and inclusions.

Optical Properties of Compositions Having Sr Substitute for Ca

The effect of varying the ratio of the two divalent elements, when thoseelements are strontium and calcium, is shown in FIG. 11, In FIG. 11 theinclusion of the second divalent metal Sr is shown increasing from leftto right along the x-axis of the graph, from a value of no strontium(x=0) to a value of 100% (where there is no calcium at x=1).

FIG. 11 contains two parts: the first is peak emission wavelength shownin FIG. 11A, and the second is photoluminescent (PL) value shown in FIG.11B. The tested samples are compounds having the general formulaCa_(0.98−x)Sr_(x)AlSiN₃Eu_(0.02). Thus, the two divalent elements thatare simultaneously present in this phosphor are calcium and strontium,with the exceptions of the endpoints, shown for comparison where onlyone metal is present. The results in FIG. 1 show that as the strontiumcontent “x” is increased from 0 to 1, the wavelength of the peakemission first increases slightly, meaning that the emission is becomingmore red (where the longest wavelength occurs at x=0.2); then decreasesgradually, from a maximum of about 660 nm (x=0.2) to final value ofabout 637 nm (at x=1). The photoluminescence in FIG. 11B is generallyabout constant between x=0 to x=0.8, but then decreases substantially asthe strontium content is increased from x=0.8 to x=0.9.

FIG. 12 is a collection of normalized emission spectra (FIG. 12A), toshow the effect of maximum emission wavelength of a high ratio of Sr toCa in the present phosphors; and emission spectra that has not beennormalized (FIG. 12B) of the same two compounds having the formulaCa_(0.98−x)Sr_(x)AlSiN₃Eu_(0.02) with x=0.82 and x=0, respectively, toshow relative brightness. The emission peak wavelength changed as muchas about 20 nm, from 658 nm to 638 nm for x=0 and x=0.82, respectively,but the x=0.82 strontium content doped samples are brighter as well.

FIG. 13 shows the effect of doping the present nitride-based redphosphors with further amounts of an alkaline earth metal; specifically,Ca, Sr, and Ba replacing a portion of the Ca/Sr mix discussed inprevious paragraphs. Here the compound under study had the formulaCa_(0.2)Sr_(0.74)AlSiM_(0.05)N₃Eu_(0.01), where M was Ca, Sr, Ba,respectively, with M as 0 being used as a control for the experiment.The europium source was the fluorinated EuF₃; likewise, alkaline earthfluorides were used as the sources of the Ca, Sr, and the Ba. This dataset shows that the peak emission wavelength shifts to longer wavelengthswith the addition of the 5% alkaline earth metal. The order of thewavelength increase was Ca, then, Sr, and finally Ba.

FIGS. 14A and 14B are a collection of emission wavelengths, the raw datashown in FIG. 14A, and a normalized version shown in FIG. 14B, for anitride-based red phosphor with a 5% level of a group IIIA (of theperiodic table) metal M, where M is in this example either boron orgallium. These doping metals (semiconductors and/or semi-metals) areshown in the following formula as substituting for the trivalent metalaluminum: Ca_(0.2)Sr_(0.79)Al_(0.95)SiM_(0.05)N₃Eu_(0.01):F. Again, EuF₃was used as the source of both the activator europium, as well as thehalogen dopant that substitutes for nitrogen. Here, the peak emissionwavelength shifted to longer wavelengths (shown best in the normalizeddata set of FIG. 14B), and the photoluminescence decreased (shown bestin the non-normalized data set of FIG. 14A), with the addition of the 5%alkaline earth metal, in the order of boron and gallium.

The Activator

FIG. 15A is a graph showing peak emission wavelength changes as afunction of Eu source and content, where Eu sources include the oxide,fluoride, and chloride salts of the europium metal. The compound chosenfor this study was Ca_(0.2)Sr_(0.8−x)AlSiN₃Eu_(x), where the parameter“x” represents the amount of the Eu activator present; its value rangingfrom 0.005 to 0.015 in this experiment. The data shows that the peakemission wavelength shifts to longer wavelengths (a deeper red color) asthe Eu content is increased, with the phosphors synthesized from thefluoride of europium showing the longest wavelengths of the group. Thephosphors synthesized from the chlorides of europium demonstrated theshortest wavelengths with the oxides in the middle of the data set: thisset shows that a particularly desired emission wavelength may beachieved via the proper selection of starting materials (in this case,the nature of the europium source).

Not only do the EuF₃ generated samples emit at longer wavelengths thanEu₂O₃ based samples having the same europium content, but the EuF₃generated samples are brighter as well. This is illustrated in FIG. 15B.The data in FIG. 15B is in a format that is similar to that of FIG. 15A,but this time photoluminescence (PL) is plotted as a function ofeuropium content. Again, data was collected for three types ofcompounds; those synthesized from the fluorides, chlorides, and oxidesof the europium metal. Here, the photoluminescence first increased asthe europium content was increased, reaching a maximum at a content ofabout 0.01. Higher than values of 0.01, values to 0.015, thephotoluminescence either leveled off, or decreased slightly for thethree variations of Ca_(0.2)Sr_(0.8−x)AlSiN₃Eu_(x). While it is notprecisely known whether this is due to the inclusion of halogen orabsence of oxygen (by a halogen-instigated oxygen gettering effect), itis recognized that either way, the effect is advantageous.

Activators that may be used in conjunction with the present rednitrides, in particular the rare earths, include Mn, La, Ce, Pr, Nd, Pm,Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. These activators may be used inany combination, and in the desired amounts, up to and including 5atomic percent.

Excitation Spectra of the Present Nitride-Based Red Phosphors

FIG. 16 is a collection of excitation spectra for the R630 compositionhaving the formula Ca_(0.2)Sr_(0.8)AlSiN₃Eu_(0.01), and the R640composition having the formula Ca_(0.158)Sr_(0.815)AlSiN₃Eu_(0.023.)Excitation spectra are taken by measuring the photoluminescence at achosen wavelength, in this case around 630 to 640 nm, as a function ofthe wavelength of the incoming excitation radiation used to cause thephosphor(s) to photoluminesce. In this experiment, the wavelength of theexcitation radiation was increased from about 400 nm, which lies in theUV to visible purple range of the electromagnetic spectrum, to about 600nm, which is orange and approaching red. The data shows that theexemplary phosphors of the present embodiments are efficient atphotoluminescing when the wavelength of the excitation radiation rangesfrom about 400 to about 550 nm (yellow).

The present phosphors may also be activated in the ultraviolet andnear-ultraviolet, at wavelengths ranging from about 250 to 400 nm.

X-Ray Diffraction (XRD) Pattern of the Present Nitride-Based RedPhosphors

An x-ray diffraction (XRD) pattern of one of the typical compositions ofthe present embodiments is shown in FIG. 17. The particular compoundexamined was Ca_(0.2)Sr_(0.79)AlSiN₃Eu_(0.01):F. It may be seen thatthis compound has the CaAlSiN₃-type structure by comparing its x-raydiffraction pattern (FIG. 17 of the present disclosure) with thepatterns of a CaAlSiN₃ host, and CaAlSiN₃ host activated with Eu (FIGS.1-1 and 1-2 of US 2007/0007494, respectively).

Nitride-based, red-emitting phosphors of the present embodiments, evenwith large amounts of Sr present and replacing Ca, may be identified bythe presence of what is perhaps the highest intensity diffraction peakat a 20 of about 36 degrees, representing diffraction from the (311)plane; another high intensity at about 35 degrees from the (002) peak; apeak at about 32 degrees from the (310) plane; a set of peaks at 40-41degrees from the (221) plane; and a set of peaks at 48 degrees from the(312) and/or (022) planes. That the intensities of the peaks in FIG. 17of the present disclosure differ from those of US 2007/0007494, withrespect to the (hkl) planes identified above, is indicative of a slightshifting of the atomic positions within the unit cell as a result of thesubstitutions and inclusions of the present compounds.

Distribution of Halogen on Nitrogen Sites, Oxygen Gettering by Halogen,and Oxygen Content

Embodiments of the present invention are directed to the fluorescence ofa nitride-based deep-red phosphor having at least one of the followingnovel features: 1) an oxygen content less than about 2 percent byweight, and 2) a halogen content of virtually any amount. Such phosphorsare particularly useful in the white light illumination industry, whichutilizes the so-called “white LED,” as well as in backlightingapplications, which also utilize white light. The selection and use of arare earth halide as a raw material source of not only the rare earthactivator for the phosphor, but also the halogen, is a key feature ofthe present embodiments. While not wishing to be bound by any particulartheory, it is believed the halogen may play a dual role in enhancing theproperties of these phosphors: by reducing the oxygen content inaddition to causing an increase in photoluminescent intensity andspectral emission.

Apart from an advantage of mechanical rigidity there is anotherconsequence of the CaAlSiN₃:Eu²⁺-type structures of the presentembodiments with two types of nitrogen sites; and that is that thedistribution of halogen dopants and/or and oxygen impurities is notrandom. Hirosaki et al. describe in U.S. Publication No. 2007/0007494how in such a situation the “N is replaced with one or two or moreelements selected from the group consisting of O and F.” But Hirosaki etal. do not say how the O and F replace N, and they do not teach thegettering abilities of the halogen that are believed by the presentinventors to reduce oxygen contamination. Using a nomenclature whereby anitrogen site corner-shared by two Al/Si tetrahedral is designated asN2, and a nitrogen vertex corner-shared by three Al/Si tetrahedra as N3,the present inventors propose that, in at least some situations, theoxygen impurity occupies an N2 site because the O²⁻ valence (2−) islower than that of the N³⁻ valence (3−). If this is so, then it standsto reason that the monovalent halogen ions F⁻ and Cl⁻ also prefer tooccupy N2 sites, rather than N3 sites, for the same reasons of freeenergy minimization. In one embodiment of the present invention, oxygenand halogen compete for the same atomic positions in the unit cell.Another way of describing the two types of nitrogen sites is that the N2site is 2-fold coordinated, and the N3 site is 3-fold coordinated.

According to embodiments of the present invention, at least half (50percent) of the halogen content (either by weight or by number) resideson N2 sites rather than N3 sites. In another embodiment, at least 80percent of the halogen content (either by weight or by number) resideson N2 sites as opposed to N3 sites. In some embodiments, the desireddistribution of halogen on the N2 relative to N3 sites may be achievedby synthesizing the phosphor according to liquid mixing methods,including liquid precipitation, and the sol gel technique. Butregardless of the synthetic method, which means that including solidstate reaction mechanisms, the present inventors believe that theirmethod of halogen inclusion is related to a low oxygen impurity content.“Low oxygen impurity content” means less than about 2 weight percentoxygen. In some embodiments, low oxygen impurity means less than about1.9 weight percent oxygen, and less than about 1.8 weight percentoxygen.

Oxygen impurity has been discussed by the present inventors in aco-pending application titled “Nitride-based red phosphors,” filed Oct.13, 2008, having application Ser. No. 12/250,400. application Ser. No.12/250,400 is hereby incorporated herein in its entirety. Thatapplication discussed U.S. Pat. No. 7,252,788 to Nagatomi et al., whodiscovered and disclosed in U.S. Pat. No. 7,252,788 that when the oxygencontent in the phosphor is large, the emission efficiency decreased (notdesirable), and the emission wavelength of the phosphor also tended toshift to a shorter wavelength side. This latter observation is alsoundesirable because most (if not all) manufacturers are attempting toadd a phosphor that is deeper in the red region (i.e., less orange oryellow) for the color rendering benefits a red phosphor offers to thewhite LED industry. Nagatomi et al. continue: the phosphor they providedincludes no oxygen in the host material, with the benefits of exhibitinga higher emission efficiency, and avoiding the shift of the emissionwavelength to the shorter wavelength side [of the spectrum].

But this is more easily stated than accomplished. Oxygen contaminationwas addressed by Nagatomi et al. in US 2006/0017365, where it is taughtthat the source is believed to be the oxygen adhering to the surface ofthe raw materials, and thus introduced at the start of the synthesis;oxygen added as a result of oxidation of the surface of the rawmaterials at the time of preparation for firing, and the actual firing,and the oxygen adsorbed onto the surface of the phosphor particles afterfiring.

A discussion of oxygen measurements, and an analysis of the possiblecauses for a discrepancy between measured and calculated values, wasalso given by Nagatomi et al. in US 2006/0017365. The oxygen contentthat was measured in their sample was 2.4 percent by weight, to becontrasted with a calculated oxygen concentration of 0.3 percent byweight. The origin of this approximately 2 percent by weight differencebetween the measured value (with its so-called “excessive oxygen”)versus the calculated amount was attributed to oxygen originallyadhering to the surface of the raw materials at the time of preparationof the firing and at the time of firing, and the oxygen adsorbed ontothe surface of the phosphor specimen after the firing.

The oxygen content in Nagatomi et al.'s samples of U.S. Pat. No.7,252,788 similarly show a 2 plus percent by weight value: 2.2, 2.2, and2.1 in Tables 1 and 3.

Hirosaki et al. (US 2007/0007494) teach a CaAlSiN₃ family of crystalphases, where the “N is replaced with one or two or more elementsselected from the group consisting of 0 and F.” In the case of oxygen,its content in a CaAlSiN₃:Eu phosphor was measured. A synthesizedcompound having the intended formula Eu_(0.0008)Ca_(0.992)AlSiN₃ waspulverized and a compositional analysis performed by ICP emissionspectrometry. The oxygen and nitrogen in the sample were measured in aLECO model TC-436 oxygen and nitrogen analyzer. The oxygen content wasdetermined to be 2.0±0.1% by weight, and its presence was attributed tooxygen impurities contained in the nitrides of the metals (Si, Al, Ca)used as starting materials. Thus the composition of the synthesizedinorganic compound, calculated from the analytical results, isEu_(0.0078)Ca_(0.9922)Si_(0.0997)Al_(0.9996)N_(2.782)O_(0.172). Thefluorine content was not measured in any of the samples.

Measured oxygen content of the present R630 phosphors, whose formulashave been given in preceeding sections, are about 1 weight percent. Themeasured oxygen content of the R640 phosphors was about 1.35 weightpercent.

Application in Backlighting and White Light Illumination Systems

According to further embodiments of the present invention, the presentred phosphors may be used in white light illumination systems, commonlyknown as “white LEDs,” and in backlighting configurations for displayapplications. Such white light illumination systems comprise a radiationsource configured to emit radiation having a wavelength greater thanabout 280 nm; and a halide anion-doped red nitride phosphor configuredto absorb at least a portion of the radiation from the radiation source,and emit light with a peak intensity in a wavelength range greater thanor equal to about 630 nm. Exemplary spectra of light intensity versuswavelength emitted by these warm white luminescent systems are shown inFIGS. 18A-18E. The properties of the present nitride-based, redphosphors are also suitable as components of a backlighting illuminationsystem.

For these applications, the present nitride-based red phosphor maycombined with any of a yellow, green, or orange emitting phosphor, usedeither singly or in any variety of combinations as desired, in aphosphor mix or package. The yellow, green, or orange phosphors may beeither aluminate or silicate-based. Exemplary yellow, yellow-green, andgreen silicate-based phosphors may be of the form M₂SiO₄, where M is adivalent metal such as an alkaline earth (e.g., Mg, Ca, Sr, and Ba,singly or in combinations), or other such elements such as Zn. Exemplaryorange silicate-based phosphors may be of the form M₃SiO₅, where M isdefined as above in this paragraph. These silicate-based orange and/orgreen phosphors may additionally contain a halogen such as F or Cl, andin some embodiments, this halogen anion substitutes for oxygen and mayreside on oxygen lattice sites in the crystal.

Exemplary silicate-based yellow-green phosphors that are particularlysuitable for combination with the present nitride-based red phosphorshave been described by the present inventors in U.S. Pat. No. 7,311,858to Wang et al. The phosphors disclosed in the patent have the formula M₂SiO₄:Eu²⁺D, where M is a divalent metal selected from the groupconsisting of Sr, Ca, Ba, Mg, Zn, and Cd, and D is a dopant selectedfrom the group consisting of F, Cl, Br, I, P, S, and N. The dopant D ispresent in the phosphor in amounts ranging from about 0.01 to about 20mole percent, and at least some of the dopant anions substitute foroxygen anions. These silicate-based phosphors are configured to absorbradiation in a wavelength ranging from about 280 nm to about 490 nm, andto emit visible light in wavelengths ranging from about 460 to about 590nm. U.S. Pat. No. 7,311,858 is incorporated herein in its entirety.

Exemplary silicate-based green emitting phosphors suitable for use withthe present red nitrides have been described by the present inventors inthe Patent Application Publication US 2006/0145123 to Cheng et al. Thesephosphors are also of the M₂SiO₄ form, specifically having the formula(Sr,A₁)_(x)(Si,A₂)(O,A₃)_(2+x):Eu²⁺, where A₁ is at least one divalentcation selected from the group consisting of Mg, Ca, Ba, or Zn, or acombination of 1+ and 3+ cations; A₂ is a 3+, 4+, or 5+ cation,including at least one of B, Al, Ga, C, Ge, N, and P; A₃ is a 1−, 2−, or3− anion, including any of F, Cl, Br, and S. In this formulation, x isany value between 1.5 and 2.5, both inclusive. The formula is written toindicate that the A₁ cation replaces Sr; the A₂ cation replaces Si, andthe A₃ anion replaces O. The amounts of the A₁, A₂, and A₃ ions (whethercationic or anionic) each range from about 0 to about 20 mole percent.Patent Application Publication US 2006/0145123 is incorporated herein inits entirety.

Exemplary silicate-based orange emitting phosphors suitable for use withthe present red nitrides have been described by the present inventors inthe Patent Application Publication US 2007/0029526 to et al. Thesephosphors are also of the M₃SiO₅ form, specifically having the formula(Sr_(1−x)M_(x))_(y)Eu_(x)SiO₅, where M is at least one of a divalentmetal selected from the group consisting of Ba, Mg, Ca, and Zn; 0<x<0.5;2.6<y<3.3, and 0.001<z<0.5, Patent Application Publication US2007/0029526 is incorporated herein in its entirety.

Specific combinations of orange-emitting silicates and the present redemitting nitrides; green-emitting silicates and the present red emittingnitrides; various combinations thereof (meaning combinations of orangeand green-emitting silicates with the present red emitting nitrides),for use in backlighting and warm white light illumination systems, willbe described next. Exemplary data is shown in FIGS. 18A-18E, and thecombinations are: a green phosphor emitting at about 525 nm (EG525) plusthe R630 red phosphor previously described (FIG. 18A); the same green EG525 nm phosphor in combination with the R640 phosphor previouslydescribed (FIG. 18B); a green phosphor emitting at about 540 and theR630 red nitride (FIG. 18C); the green EG540 phosphor in combinationwith the red R640 phosphor (FIG. 18D), and finally, a three-componentsystem having a green phosphor EG540, an orange phosphor emitting at 586nm (designated 0586), and the red R640 nitride (FIG. 18E). Each of thered nitride phosphor compositions may have oxygen contents that are lessthan or equal to about 2 weight percent.

The compositions of the red nitrides used in the examples of FIGS.18A-18E are as follows: the R630 phosphor has the formulaCa_(0.2)Sr_(0.8)AlSiN₃Eu_(0.01):F, and the R640 phosphor has the formulaCa_(0.158)Sr_(0.815)AlSiN₃Eu_(0.023):F. The green phosphor EG540 is thechlorinated version of an M₂SiO₄ silicate, and has the formulaSr_(0.925)Ba_(0.025)Mg_(0.05)Si_(1.03)(O,Cl)₄:Eu²⁺. The EG540 greenphosphor is also chlorinated, having the formulaSr_(1.15)Ba_(0.8)Mg_(0.05)Si_(1.03)(O,Cl)₄:Eu²⁺. The O586 orangephosphor is a fluorinated version of the M₃SiO₅ silicate, and thisparticular phosphor has the formula Sr₃Eu_(0.06)Si_(1.02)O₅F_(0.18).

Each of the figures gives the CIE x and y coordinates for the whitelight illumination resulting from these exemplary combinations, thecolor coordinated temperature (CCT), the brightness, and the colorrendering index R_(a). Although specific examples of yellow, green, andorange phosphors that may be used in conjunction with the presentnitride-based red phosphors, this is not to say the present red nitridesare restricted to use with those phosphors. According to embodiments ofthe present invention, the present nitride-based red phosphors (having adesired distribution of the halogen over N2 and N3 sites, and/or havingan oxygen content less than about 2 weight percent), may be used withvirtually any blue, yellow, green, orange, or other type of redphosphor, regardless of structure or chemical makeup, in the white LEDand backlighting display arts.

Many modifications of the exemplary embodiments of the inventiondisclosed above will readily occur to those skilled in the art.Accordingly, the invention is to be construed as including all structureand methods that fall within the scope of the appended claims.

What is claimed is:
 1. A nitride-based, red emitting compound of the type (Ca,Sr)AlSiN₃:Eu²⁺, wherein the impurity oxygen content is less than about 2 percent by weight.
 2. A nitride-based, red emitting compound of the type (Ca,Sr)AlSiN₃:Eu²⁺, further comprising a halogen whose content ranges from greater than about zero to about 2 atomic percent.
 3. The compound of claim 2, wherein the halogen is selected from the group consisting of F and Cl.
 4. The compound of claim 2, wherein at least half of the halogen is distributed on 2-fold coordinated nitrogen (N2) sites relative to 3-fold coordinated nitrogen (N3) sites.
 5. An RGB lighting system for use in a white LED or backlighting display application, the RGB lighting system comprising: the nitride-based, red-emitting phosphor according to claim 1, a green phosphor, and a blue LED emitting in a wavelength ranging from about 400 nm to about 550 nm.
 6. An RGB lighting system for use in a white LED or backlighting display application, the RGB lighting system comprising: the nitride-based, red-emitting phosphor according to claim 2, a green phosphor, and a blue LED emitting in a wavelength ranging from about 400 nm to about 550 nm. 